(1) Field of the Invention
The present invention relates to a field-effect transistor (FET), especially to an FET for use in a high-frequency communication device.
(2) Related Art
Conventionally, a Metal-Insulator-Semiconductor Field-Effect Transistor (MISFET), which is an insulated-gate FET, has been utilized in an electronic device such as a silicon semiconductor integrated circuit device.
When compared with a silicon (Si) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) of the same size, an FET formed using gallium arsenide (GaAs) or a combination of GaAs and aluminum gallium arsenide (AlxGa1-xAs) provides higher-speed performance, and is more suitable for a microwave amplifier and a high-speed logic IC. This is because group III–V compound semiconductors such as GaAs, InP and GaN have a higher (approximately five- to six-fold) electron mobility than Si. An FET formed using a semiconductor with such a high electron mobility achieves excellent high-frequency characteristics.
However, a MOSFET or MISFET formed using a group III–V compound semiconductor such as GaAs has a problem regarding interface states. To be specific, interface states are formed at an interface between an insulating film and a compound semiconductor. Because such interface states lower mutual conductance (gm), the MOSFET or MISFET can not achieve good performance.
For this reason, a Metal Semiconductor Field-Effect Transistor (MESFET) or a High Electron Mobility Transistor (HEMT) formed using a compound semiconductor such as GaAs is used for a constituent of an electronic device such as a high-frequency amplifier and an ultrahigh-speed integrated circuit device. For example, a MESFET is used as an amplifying element in a mobile communication device such as a mobile telephone, and a HEMT is used as an amplifying element in a receiving antenna for satellite broadcasting.
In particular, a HEMT provides high-speed performance and low noise figure, since two-dimensional electron gas (2DEG) which is produced in a channel layer made of high-purity InGaAs and has a high electron mobility is used as carriers. To be specific, a layer in which electrons are generated (carrier supply layer) and a layer in which electrons move (the channel layer) are separated from each other. Therefore, electrons can move stably at high speed from a source to a drain without colliding with impurities, thereby reducing noise figure.
Here, to increase a surface charge density of the 2DEG accumulated in the channel layer, an energy barrier formed at an interface between the channel layer and the carrier supply layer needs to be made as high as possible.
In the HEMT, a Schottky contact layer and an ohmic contact layer are formed, in the stated order, on the carrier supply layer during a wafer epitaxial growth. The ohmic contact layer is partially removed, so as to partially expose the Schottky contact layer. A gate electrode is provided on the exposed surface of the Schottky contact layer. Furthermore, a source electrode and a drain electrode are formed on the ohmic contact layer so as to oppose each other with the gate electrode therebetween.
As described above, the Schottky contact layer, on which the gate electrode is formed, is sandwiched between the carrier supply layer and the ohmic contact layer. Here, a high Schottky barrier is formed between the gate electrode and the Schottky contact layer. A height ΦB of the Schottky barrier is defined as a difference between an electron affinity φs of the Schottky contact layer and an electron affinity φm of a metal forming the gate electrode, as indicated by the following formula (1):ΦB=φm−φs  (1)
If the Schottky barrier ΦB is sufficiently high, electrons in the carrier supply layer and the 2DEG accumulated in the channel layer are difficult to overcome or pass through the Schottky barrier ΦB to flow into the gate electrode. This can reduce leakage currents caused when a reverse bias is applied, and improve resistance to dielectric breakdown.
The Schottky contact layer forms a path of electric currents flowing between the source electrode and the drain electrode. In detail, electrons need to go through the Schottky contact layer in order to reach the channel layer from the source electrode, and in order to reach the drain electrode through the channel layer. Therefore, while forming a high Schottky barrier (high resistance) with respect to the gate electrode as stated above, the Schottky contact layer is required to provide a lowest possible resistance with respect to the source electrode and the drain electrode. To meet these contradictory needs, the ohmic contact layer is formed between the Schottky contact layer and the source and drain electrodes. As a result, an ohmic contact, which has a low resistance, is formed between each of the source and drain electrodes and the ohmic contact layer. Hence, a resistance between the source and drain electrodes is reduced.
Furthermore, the Schottky contact layer is conventionally made of the same material as the carrier supply layer, which is AlGaAs. Therefore, the Schottky barrier ΦB is relatively high in the HEMT including the Schottky contact layer made of AlGaAs. This results in reduction in leakage currents caused when the gate electrode is reverse-biased.
However, the HEMT including the Schottky contact layer made of AlGaAs has a problem of frequency dispersion of drain currents. In other words, a maximum drain current density decreases when a high-frequency signal is applied. Frequency dispersion of drain conductance causes a degree of amplification of a circuit to vary in accordance with an operating frequency. This makes it impossible to amplify a pulse appropriately.
A possible reason for such frequency dispersion of drain currents is that AlGaAs has a high interface state density. AlGaAs forms many interface states, that is to say, electron trap levels, at an interface between different layers. Electrons are trapped by the electron trap levels at the interface. Each of the electron trap levels releases trapped electrons into a conduction band after a lifetime of the electron trap level. Here, the lifetime of the interface states formed by AlGaAs is relatively long. If an FET with such many electron trap levels having a long lifetime operates at a frequency corresponding to the lifetime, the electron trap levels significantly affects frequency performance of the FET.
For example, some electron trap levels of a GaAs substrate have a long lifetime of several milliseconds. If an operating frequency of several KHz is set as a criteria, there is a large difference in performance of the FET between when operating at frequencies lower and higher than the criteria. This causes frequency dispersion of drain conductance.
The problem of the frequency dispersion can be solved by a technique disclosed in Japanese patent application H15-032038. This technique uses InGaP for a Schottky contact layer, instead of AlGaAs.
FIG. 5 illustrates a cross-section of an FET 50 disclosed in the patent document. As shown in FIG. 5, the FET 50 is formed by providing an AlGaAs buffer layer 61, an n-AlGaAs barrier layer 62, an undoped InGaAs channel layer 63, an n-AlGaAs carrier supply layer 64, an undoped InGaP Schottky contact layer 65, and an ohmic contact layer 66 which has an opening so that part of the Schottky contact layer 65 is exposed, in the stated order, on a GaAs semiconductor substrate 60.
Furthermore, a gate electrode 67 is formed so as to be in contact with the Schottky contact layer 65, and a source electrode 68 and a drain electrode 69 are each formed so as to be in contact with the ohmic contact layer 66.
The Schottky contact layer 65 is made of InGaP, which has a lower interface state density than AlGaAs. Therefore, the number of interface states formed at an interface between the Schottky contact layer 65 and the gate electrode 67 can be reduced. This can reduce frequency dispersion of drain currents.
However, the conventional FET 50 has the following drawback. A discontinuity in energy of a conduction band (hereinafter referred to as a band discontinuity) occurs at an interface between the AlGaAs carrier supply layer 64 and the InGaP Schottky contact layer 65. This band discontinuity blocks electrons passing through the interface between the carrier supply layer 64 and the Schottky contact layer 65. Therefore, a source resistance Rs of the FET 50 increases, and mutual conductance (gm), which is one of the properties of the FET 50, accordingly decreases. As a result, the FET 50 can not obtain a sufficient current drive power.
It is known that the amount of a band discontinuity at an interface between different layers is generally determined by materials of the layers, and that, if the layers are each made of mixed crystal, a composition of each of the layers determines the amount of the band discontinuity. Here, the mutual conductance (gm) is calculated by the following formula (2):gm=gmi/(1+Rs×gmi)  (2)
In the formula (2), gm, gmi, and Rs respectively denote mutual conductance, intrinsic mutual conductance, and a source resistance. If the carrier supply layer 64 is made of Al0.25Ga0.75As, as an example, a band discontinuity of approximately 0.3 eV occurs at the interface between the AlGaAs carrier supply layer 64 and the InGaP Schottky contact layer 65, as shown in FIG. 6.
In FIG. 6, an energy level of a conduction band is plotted on the vertical axis, and the layers composing the FET 50 are plotted on the horizontal axis in the order of the lamination on the substrate 60.